1. Technical Field of the Invention
The present invention relates to a process for producing hexafluoroethane, comprising a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom and a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound with fluorine gas in the gaseous phase in the presence of a diluting gas, and also relates to the use thereof.
2. Background Art
Pentafluoroethane (hereinafter referred to as xe2x80x9cCF3CHF2xe2x80x9d) is used, for example, as a refrigerant for low-temperature use or a starting material for the production of hexafluoroethane (hereinafter referred to as xe2x80x9cCF3CF3xe2x80x9d).
For the production of CF3CHF2, for example, the following methods have been heretofore known:
(1) a method of fluorinating perchloroethylene (CCl2xe2x95x90CCl2) or a fluoride thereof with hydrogen fluoride (see, JP-A-5-97724 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-6-506221, JP-A-7-76534, JP-A-7-118182, JP-A-8-268932 and JP-A-9-511515),
(2) a method of performing hydrogenolysis of chloropentafluoroethane (CClF2CF3), and
(3) a method of reacting a fluorine gas with a halogen-containing ethylene (see, JP-A-1-38034).
When these methods for producing CF3CHF2 are used, the objective CF3CHF2 contains a compound having chlorine atom within the molecule as main impurities. The compound having chlorine atom within the molecule includes a compound having one carbon atom within the molecule, such as chloromethane, chlorodifluoromethane and chlorotrifluoromethane, a compound having two carbon atoms within the molecule, such as chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane, and an unsaturated compound such as chlorotrifluoroethylene.
In the case of producing CF3CF3 by a direct fluorination reaction of reacting CF3CHF2 with a fluorine gas (F2), if CF3CHF2 contains the compound having chlorine atom within the molecule, chlorine, hydrogen chloride, chlorine fluoride or different kinds of chlorofluorocarbons are generated in the reaction with fluorine gas. Even when hydrofluorocarbons (HFC) or perfluorocarbons (PFC) are contained in CF3CHF2, there arises no particular problem, however, for example, chloromethane (CH3Cl) or chlorodifluoromethane (CHClF2) reacts with fluorine gas to produce chlorotrifluoromethane (CClF3). The objective CF3CF3 and chlorotrifluoromethane form an azeotropic composition, therefore, CClF3 is difficult to remove even by performing distillation, adsorption for purification, or the like. Accordingly, in the case of reacting CF3CHF2 with a fluorine gas to produce CF3CF3, the amount of the compound having chlorine atom within the molecule contained in CF3CHF2 should be reduced as much as possible.
According to conventional production methods for CF3CHF2, the total amount of the compound having chlorine atom within the molecule is sometimes as high as about 1 vol %. Therefore, a distillation operation is repeated for removing these compounds contained in CF3CHF2 and elevating the purity of CF3CHF2, however, this has such a problem that the distillation cost increases, the distillation loss is caused, the profitability is bad and some compounds having chlorine atom within the molecule form an azeotropic mixture or an azeotrope-like mixture with CF3CHF2 and are very difficult to remove only by the distillation operation. In particular, chloropentafluoroethane (hereinafter referred to as xe2x80x9cCClF2CF3xe2x80x9d) is usually contained in CF3CHF2 in a concentration of thousands of ppm or more but since an azeotropic mixture is formed by CF3CHF2 and CClF2CF3, the separation is hardly attained by distillation which is a commonly used separation and purification method.
For separating CClF2CF3 contained in CF3CHF2, various methods have been proposed, for example,
(1) a method of adding a third component to a mixture of CF3CHF2 and CClF2CF3 and performing the extractive distillation (see, JP-A-6-510980, JP-A-7-133240, JP-A-7-258123, JP-A-8-3082, JP-A-8-143486 and JP-A-10-513190),
(2) a method of removing CClF2CF3 contained in CF3CHF2 using an adsorbent (see, JP-A-6-92879 and JP-W-8-508479 (the term xe2x80x9cJP-Wxe2x80x9d as used herein means an xe2x80x9cunexamined published international patent applicationxe2x80x9d)), and
(3) a method of converting CClF2CF3 contained in CF3CHF2 into CF3CHF2 in the presence of a hydrogenation catalyst (see, JP-A-7-509238, JP-A-8-40949, JP-A-8 -301801 and JP-A-10-87525).
However, these methods have a problem, that is, the method of (1) requires a step of recovering the third component from the mixture of CClF2CF3 and the third component, the method of (2) requires a step of regenerating the adsorbent, and the method of (3) suffers from reduction in the catalytic life due to hydrogen chloride produced.
The present invention has been made under these circumstances and the object of the present invention is to provide a method for producing CF3CF3 with good profitability using a gas mixture containing CF3CHF2 and a compound having chlorine atom within the molecule in the method for producing CF3CF3 which is used as an etching or cleaning gas in the process of producing a semiconductor device, and also provide a use thereof.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that in the method for producing CF3CF3, when a gas mixture containing CF3CHF2 and a compound having chlorine atom within the molecule as impurities is reacted with hydrogen fluoride in the presence of a fluorination catalyst to convert CClF2CF3 which is contained in the gas mixture, into CF3CF3 and then performing a direct fluorination reaction of reacting the resulting gas mixture containing CF3CHF2 and CF3CF3 with a fluorine gas in the gaseous phase in the presence of a diluting gas, the above-described problems can be solved. The present invention has been accomplished based on this finding. The present invention provides a process for producing CF3CF3 and use thereof, described in [1] to [19] below.
[1] A process for producing hexafluoroethane, comprising the following two steps:
(1) a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom; and
(2) a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound obtained in the step (1) with a fluorine gas in the gaseous phase in the presence of a diluting gas.
[2] The process for producing hexafluoroethane as described in [1], wherein the compound having chlorine atom is at least one compound selected from the group consisting of chloromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane, chlorotrifluoroethane and chlorotrifluoroethylene.
[3] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 1 vol % or less.
[4] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 0.5 vol % or less.
[5] The process for producing hexafluoroethane as described in any one of [1j to [4], wherein in the step (1), the fluorination catalyst is a bulk catalyst obtained by adding indium to an oxide of chromium.
[6] The process for producing hexafluoroethane as described in any one of [1] to [5], wherein in the step (1), the temperature at the reaction with hydrogen fluoride in the presence of a fluorination catalyst is in the range of 150 to 480xc2x0 C.
[7] The process for producing hexafluoroethane as described in any one of [1] to [6], wherein in the step (1), the molar ratio of hydrogen fluoride/organic substance contained in the gas mixture is in the range of 0.5 to 5.
[8] The process for producing hexafluoroethane as described in any one of [1] to [7], wherein a step of removing an acid content containing hydrogen chloride produced is conducted before the step (2).
[9] The process for producing hexafluoroethane as described in any one of [1] to [8], wherein a step of separating chlorotetrafluoroethane and/or chlorotrifluoroethane, and returning the chlorotetrafluoroethane and/or chlorotrifluoroethane separated to the step (1) is conducted before the step (2).
[10] The process for producing hexafluoroethane as described in any one of [1] to [9], wherein in the step (2), the total amount of the compound having chlorine atom contained in the gas mixture is 0.02 vol % or less.
[11] The process for producing hexafluoroethane as described in any one of [1] to [10], wherein in the step (2), the fluorinated compound contained in the gas mixture is mainly composed of hexafluoroethane.
[12] The process for producing hexafluoroethane as described in any one of [1] to [11], wherein in the step (2), the diluting gas is a gas containing at least one selected from the group consisting of tetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogen fluoride.
[13] The process for producing hexafluoroethane as described in any one of [1] to [12], wherein in the step (2), the diluting gas is a gas rich in hydrogen fluoride.
[14] The process for producing hexafluoroethane as described in any one of [1] to [13], wherein in the step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 250 to 500xc2x0 C.
[15] The process for producing hexafluoroethane as described in any one of [1] to [14], wherein in the step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 350 to 450xc2x0 C.
[16] A hexafluoroethane product comprising hexafluoroethane having a purity of 99.9997 vol % or more.
[17] The hexafluoroethane product as described in [16], wherein the content of the compound having chlorine atom is 1 volppm or less and the content of the pentafluoroethane is 1 volppm or less.
[18] An etching gas comprising the hexafluoroethane product described in [16] or [17].
[19] A cleaning gas comprising the hexafluoroethane product described in [16] or [17].
In summary, the present invention provides xe2x80x9ca process for producing CF3CF3, comprising a step of reacting a gas mixture containing CF3CHF2 and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom and a step of reacting a gas mixture containing CF3CHF2 and the fluorinated compound obtained by the above-described step with a fluorine gas in the gaseous phase in the presence of a diluting gasxe2x80x9d, xe2x80x9can CF3CF3 product comprising CF3CF3 having a purity of 99.9997 vol % or morexe2x80x9d, xe2x80x9can etching gas comprising the above-described CF3CF3 productxe2x80x9d and xe2x80x9ca cleaning gas comprising the above-described CF3CF3 productxe2x80x9d.
The production process for CF3CF3 and use thereof according to the present invention are described in detail below.
As described above, CF3CHF2 for use in the present invention is generally produced by fluorinating perchloroethylene (CCl2xe2x95x90CCl2) or a fluoride thereof with hydrogen fluoride (HF), and CF3CHF2 contains a compound having chlorine atom derived from the starting material, such as chloromethane, chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane. In order to purify CF3CHF2 containing these compounds to a high purity, known methods by a distillation operation are employed, however, these methods have such a problem that these are not economical since the compound and CF3CHF2 form an azeotropic mixture or an azeotrope-like mixture, the purification by separation is very difficult, the number of stages of the distillation tower or the number of the distillation towers must be increased, and the cost for equipment or energy increases.
In the present invention, the compound having chlorine atom contained in CF3CHF2 as impurities is fluorinated with hydrogen fluoride at an elevated temperature in the presence of a fluorination catalyst and thereby converted into hydrofluorocarbon (HFC) or perfluorocarbon (PFC). For example, in fluorinating CClF2CF3 or chlorotetrafluoroethane contained as impurities in CF3CHF2 using hydrogen fluoride, a reaction shown by the following formula (1) or (2) takes place:
CF3CClF2+HFxe2x86x92CF3CF3+HClxe2x80x83xe2x80x83(1)
CF3CHClF+HFxe2x86x92CF3CHF2+HClxe2x80x83xe2x80x83(2)
The product is HFC or PFC free of chlorine atom, and hydrogen chloride is produced as a by-product.
In the present specification, the gas mixture containing CF3CHF2 and the compound having chlorine atom is sometimes referred to as xe2x80x9cstarting gas mixturexe2x80x9d.
In this fluorination reaction, the compound which is converted into HFC or PFC is chloromethane, chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane. These compounds are usually contained in CF3CHF2 in a total amount of thousands of ppm or more. When the starting gas mixture containing these compounds is reacted with a fluorine gas, the methane-type compounds are converted into CClF3 and the ethane-type compounds are converted into CClF2CF3, therefore, CF3CF3 obtained after the reaction contains CClF3 and CClF2CF3 as main impurities.
CClF2CF3 scarcely reacts with a fluorine gas at low temperatures. However, according to the investigations by the present inventors, for example, at a reaction temperature of 400xc2x0 C., the amount of CClF3 produced by the decomposition of CClF2CF3 is 1 ppm or less when the concentration of CClF2CF3 contained in the starting gas mixture is about 800 ppm or less, and about 2 ppm of CClF3 is produced when the concentration of CClF2CF3 exceeds about 2,000 ppm. CClF3 forms an azeotropic mixture with CF3CF3, therefore, even if the concentration is low, this compound is difficult to remove by an operation of distillation, adsorption for purification or the like. Accordingly, it is preferred that not only a compound which produces CClF3 upon reaction with a fluorine gas is removed from CF3CHF2 as a starting material but also the CClF2CF3 content is reduced to a low concentration as much as possible.
The total amount of the compound having chlorine atom contained in the starting gas mixture for use in the present invention is preferably 1 vol % or less, more preferably 0.5 vol % or less, still more preferably 0.3 vol % or less. If the concentration of the compound having chlorine atom exceeds 1 vol %, the reaction must be performed at a high temperature and the life of the fluorination catalyst is disadvantageously shortened, moreover, a side reaction proceeds at the same time and the productivity decreases.
The fluorination catalyst comprises at least one element selected from the group consisting of chromium, nickel, zinc, indium and garium, and may be a known catalyst such as supported catalyst or bulk catalyst.
In the case of the supported catalyst, carrier is preferably an alumina and/or partially fluorinated alumina, and supporting ratio is preferably 30 wt % or less. In the case of the bulk catalyst, particularly preferred is those containing chromium as main component, and having atomic ratio of nickel, zinc, indium and/or garium to chromium of 0.01 to 0.6. In the present invention, most preferred is a bulk catalyst obtained by adding indium to an oxide of chromium.
In the step of fluorinating the compound having chlorine atom, the reaction temperature is preferably from 150 to 480xc2x0 C. If the reaction temperature exceeds 480xc2x0 C., the reaction is adversely affected, for example, the catalyst deteriorates or a side reaction proceeds, and this is not preferred. Although it may vary depending on the concentration of the compound contained in the starting gas mixture, a preferred reaction temperature can be selected according to the kind of the compound. For example, in the reaction of CClF2CF3 shown in formula (1), the reaction temperature is preferably 400xc2x0 C. or more, and in the reaction of CF3CHClF shown by formula (2), the reaction temperature is preferably 300xc2x0 C. or more.
In the case of a reaction of chlorodifluoromethane (CHClF2) with hydrogen fluoride, a reaction shown by the following formula (3) takes place:
CHClF2+HFxe2x86x92CHF3+HClxe2x80x83xe2x80x83(3)
In this reaction, the reaction temperature is preferably 150xc2x0 C. or more and if the reaction temperature exceeds 400xc2x0 C. or more, a reverse reaction disadvantageously proceeds.
In the step of fluorinating a compound having chlorine atom, the reaction temperature sometimes varies depending on the kind of the compound as described above. Accordingly, in the case where a plurality of compounds are contained and these are different from each other in the optimal reaction temperature region or the concentration of each compound is high, two or more units of reactors are preferably used, though one unit of a reactor is usually sufficient.
The amount of HF used is, in terms of the molar ratio to the organic substance of the starting gas mixture containing CF3CHF2 (HF/organic substance), suitably from 0.5 to 5, preferably from 0.5 to 2. If the molar ratio is less than 0.5, the reaction is hard to proceed, whereas if it exceeds 5, a large reactor is necessary and this is not profitable.
Furthermore, in the step of fluorinating a compound having chlorine atom, the reaction pressure is preferably from atmospheric pressure to 1.5 MPa. If it exceeds 1.5 MPa, the apparatus is disadvantageously required to have pressure resistance or the like.
In the present invention, the reaction with hydrogen fluoride is performed in the presence of a fluorination catalyst using the above-described reaction conditions, and then CF3CHF2, chlorine atom-free impurities mainly comprising HFC or PFC, and hydrogen chloride as a by-product are contained in the reaction product. In the case of CF3CHF2, as the reaction temperature becomes higher, a side reaction with hydrogen chloride more proceeds as shown in the following formula (4):
CF3CHF2+HClxe2x86x92CF3CHClF+HFxe2x80x83xe2x80x83(4)
In the case of containing 1,1,1,2-tetrafluoroethane, a side reaction with hydrogen chloride more proceeds as shown in the following formula (5):
CF3CH2F+HClxe2x86x92CF3CH2Cl+HFxe2x80x83xe2x80x83(5)
Therefore, after the fluorination step of (1), the acid content containing hydrogen chloride produced is preferably removed.
The acid content is removed so as to remove unreacted hydrogen fluoride (excess hydrogen fluoride) and hydrogen chloride as a by-product. Hydrogen fluoride brings about no adverse effect in the direct fluorination reaction step but hydrogen chloride is preferably removed because this product sometimes causes an adverse effect such as production of a chlorine-containing compound or chlorine fluoride as shown in the formula (4) or (5). The step of removing the acid content is performed before the direct fluorination reaction step. Examples of the method for removing the acid content includes:
(1) in the case of containing a large amount of unreacted hydrogen fluoride, a method of introducing an effluent containing the acid content into a distillation tower, extracting hydrogen chloride from the top and extracting organic substance and hydrogen fluoride from the bottom,
(2) a method of contacting the hydrogen chloride produced and unreacted hydrogen fluoride with a purifying agent, and
(3) a method of washing the acid content with water or alkali water.
In the present invention, the method for removing the acid content is not particularly limited and, for example, the method of (3) may be used. The alkali used therein may be an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution or the like. The absorbed hydrogen fluoride may be recovered and reused, and the gas passed through the washing solution is dehydrated using a dehydrating agent such as zeolite.
The gas mainly comprising CF3CHF2 passed through the acid content-removing step sometimes contains as impurities HCFC or CFC which is not completely fluorinated by the reaction with hydrogen fluoride, and in such a case, HCFC or CFC is preferably removed by distilling before the direct fluorination reaction step.
CF3CHF2 and main compounds which may be contained in CF3CHF2 are shown, together with respective boiling points in Table 1.
The gas mainly comprising CF3CHF2 is introduced into a distillation tower, then CF4, CHF3, CF3CF3, CF3CHF2 and CClF2CF3 as the low boiling fraction are extracted from the top of the distillation tower, and CF3CHClF and CF3CH2Cl as the high boiling fraction are extracted from the bottom. The high boiling fraction extracted from the bottom is circulated into the reaction with hydrogen fluoride of the step (1). Here, the total amount of the compound having chlorine atom, which is contained in the distillate mainly comprising CF3CHF2 extracted from the top, is preferably 0.02 vol % or less. The distillate mainly comprising CF3CHF2 is used as a starting material in the direct fluorination reaction with fluorine gas.
The step (2) of reacting the gas mainly comprising CF3CHF2 with fluorine gas is described below.
The step (2) is performed in the presence of a diluting gas and the gas mainly comprising CF3CHF2 is set to a concentration lower than the explosion range. Specifically, the CF3CHF2 concentration at the reactor inlet is preferably set to about 6 mol % or less. The diluting gas is a gas containing at least one selected from the group consisting of tetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogen fluoride, preferably a diluting gas rich in hydrogen fluoride.
The amount of fluorine gas used is, in terms of the molar ratio to CF3CHF2 (F2/CF3CHF2), suitably in the range of 0.5 to 2, preferably in the range of 0.9 to 1.3. The reaction temperature is in the range of 250 to 500xc2x0 C., preferably in the range of 350 to 450xc2x0 C. If the reaction temperature exceeds 500xc2x0 C., the objective CF3CF3 is disadvantageously cleaved to produce CF4 and in the case of containing CClF2CF3 as an impurity, CClF3 is disadvantageously produced due to cleavage of CClF2CF3, whereas if it is less than 250xc2x0 C., the reaction slowly proceeds and this is not preferred.
The method for purifying the gas distilled out from the reaction step of (2) is not particularly limited. The remaining unreacted fluorine gas may be removed by adding, for example, trifluoromethane as HFC and then the residue is distilled to separate, for example, hydrogen fluoride and organic substance. The separated hydrogen fluoride is reused as the diluting gas in the direct fluorination reaction of the step (2) but may also be used as a starting material in the fluorination reaction of (1). The composition of the organic substance separated greatly differs depending on the diluting gas used for the reaction and in the case of using a gas rich in hydrogen fluoride or in the objective CF3CF3, the organic substance obtained contains CF3CF3 as a main component. In the case of using tetrafluoromethane or octafluoropropane as the diluting gas, the gas is purified by again performing distillation. In either case, high-purity CF3CF3 can be obtained by repeatedly performing the distillation operation according to the compositional ratio of the organic substance obtained.
In the distillation for purification of the organic substance, although it may vary depending on the compositional ratio, for example, an inert gas and CF4 as the low boiling fraction are extracted from the top of the first distillation tower and the gas mainly comprising CF3CF3 is extracted from the bottom and introduced into the second distillation tower. Then, an inert gas and trifluoromethane as the low boiling fraction are extracted from the top of the second distillation tower and the gas mainly comprising CF3CF3 is extracted from the bottom and introduced into the third distillation tower to extract high-purity CF3CF3 from the top, thereby performing the purification. The gas containing CClF2CF3 collected from the bottom in the third distillation may be circulated into the reaction step with hydrogen fluoride of (1).
The thus-purified CF3CF3 contains almost no impurities and high-purity CF3CF3 can be obtained. The purity thereof is 99.9997 vol % or more, and 1 volppm or less of the compound having chlorine atom and 1 volppm or less of pentafluoroethane are contained as impurities.
As the analysis method of CF3CF3 having a purity of 99.9997 vol % or more, gas chromatography (GC) using TCD method, FID method (each including the precut method) or ECD method, or an instrument such as gas chromatography mass spectrometer (GC-MS) may be used.
Use of CF3CF3 obtained by the production process of the present invention is described below.
The high-purity CF3CF3 can be used as an etching gas at the etching step in the process of manufacturing a semiconductor device and also can be used as a cleaning gas at the cleaning step in the process of manufacturing a semiconductor device.
In the process of manufacturing a semiconductor device such as LSI and TFT, a thin or thick film is formed using CVD, sputtering or vapor deposition, and the film is etched to form a circuit pattern. In the apparatus for forming a thin or thick film, cleaning for removing unnecessary deposits accumulated on the inner wall of the apparatus, jigs and the like is performed, because the produced unnecessary deposits cause generation of particles and must be removed on occasions so as to produce a film having good quality.
The etching process using CF3CF3 can be performed under various dry etching conditions such as plasma etching and microwave etching, and CF3CF3 may be used by mixing it with an inert gas such as He, N2 and Ar or with a gas such as HCl, O2 and H2 at an appropriate ratio.